1. Because it's Monday.
2. You just need a "personal admin" day to sort out annoying bank stuff or oddjobs around the house - the kind of stuff you don't want to do on a "real" day off.
3. Because your significant other also has the day off.
4. Not unrelated to the last reason, you want a day of sneaky sex. Or, at least a morning of bonking, and then a nice nap.
5. Because you can't bear the thought of another interminable meeting, or boring conversation with that colleague.
6. You're expecting a delivery of an important package.*
*Not a euphemism for Reason #4.
7. Your electric blanket is being especially clingy, and needs you to lie within its glorious heat all day.
8. You genuinely thought today was a Bank Holiday.
9. You peeked at your diary schedule for the day and it's just too much.
10. Colleagues have taken your food from the canteen fridge that one too many times, and you feel today might be the day you snap.
11. Because it's Tuesday.
12. It's too dark out.
13. You went too hard at it at the weekend, and "Day Two/Day Three Hangover" is a real medical condition.
14. Your housemates are away, so you want to avail of the free house to run wild and free. Yes, that means doing chores/dancing around while nekked.
15. You have a very important protest to attend.
16. You have some new bed linen that you really should "break in".
17. Because it's Wednesday.
18. It's too cold.
19. You've come down with a mysterious ailment that defies all medicine/anatomical science.
20. Because you woke up in a bad mood, and you just knew you were going to be a thundering bollocks for the day. Best to spare everyone that.
21. The first thing you heard this morning was the sound of rain pelting against the window, and thought, "Fuck that for a bag of chips".
22. Your office air conditioning system has two settings: extra hot and extra cold. And you/your immune system can't take it anymore.
23. Because it's Thursday.
24. You just got paid and you want to treat yo'self.
25. You can't be dealing with that skeezy co-worker who won't stop attempting to flirt with you.
26. Because it's Friday. Long weekend, whoop!
27. Well, your Netflix queue isn't going to watch itself is it? You really need to make a start on it.
28. You can feel a scratch at the back of your throat. Best not to risk it turning into a full-on throat infection.
29.You have a gigantic, scummy love bite on your neck.
30. You need respite from other commuters.
31. You were kept awake all night by foxes having loud sex.
32. You have a massive spot/cold sore.
33. There's no hot water for a shower.
34. Bad hair day.
35. Your dog/cat badly needs some TLC today due to "personal issues".
36. You're down to your last few quid before payday, so you quite literally can't afford to leave the house.
37. There was an unmissable educational segment on breakfast television that required your full attention for the best part of the day.
38. You have a lot of leftover food in the fridge that must be eaten by the end of the day, and you couldn't possibly haul it all in to work with you.
39. Your back is sore, and the cheap-ass office seats will only make it worse. So, in a way, it's all your company's fault and they owe you.
40. There was no coffee at home.
41. You got into the bath this morning, and it was simply too warm and sudsy and relaxing to get out.
42. You fancy a day hiding out at the cinema - especially where you just pay once, and then sneak between all the different screening theatres.
43. You're trying to book fiercely in-demand tickets for an event, and work will just get in the way.
44. You have a very urgent plumbing problem (keep this purposefully vague as to whether you're talking about the domestic or personal variety).
45. You have too much reading to catch up on.
46. A relative/old pal is in town, so the only civilised thing to do is to hit the pub for the day. Few things are as thrilling as illicit pints on a weekday afternoon.
47. It's the first day of the season with decent weather, and you want to be free to enjoy it, damnit!
48. You're waiting on an engineer to connect your broadband. Because, let's face it, we seem to spend 65% of our lives waiting for engineers to connect our broadband. No boss will dispute this.
49. You might be, y'know, genuinely sick.
50. If all else fails, think of it this way: even if you are legitimately ill, your boss probably won't believe you. So just go for it.